The Friend leukemia complex has been biologically separated into a helper component which is responsible for replication and a defective virus which is responsible for transformation or erythroid leukemia in the animal. The virus complex has been prepared in several in vitro tissue culture cells, a feat which has not yet been performed. The propagation in cell culture has allowed the preparation of stocks of Friend leukemia virus from which cDNA probes have been prepared to identify the erythroleukemia-inducing genetic information. The genetic information coding for erythroleukemia will be studied as to its expression in naturally occurring leukemia and the origin of this information.